The present invention relates to the field of separating and identifying microorganisms, particularly infectious agents, using two-dimensional centrifugation and exposure to chemical and enzymatic agents, combined with detection in density gradients based on light scatter or fluorescence, counting by fluorescence flow cytometry, and characterization of intact virions, bacteria, proteins and nucleic acids by mass spectrometry, flow cytometry and epifluorescence microscopy.
The publications and other materials used herein to illuminate the background of the invention or provide additional details respecting the practice, are incorporated by reference, and for convenience are respectively grouped in the appended List of References. Patents referenced herein are also incorporated by reference.
In the prior art, diagnosis of viral and bacterial infections has been done by culturing the causal agents in suitable media or in tissue culture to obtain sufficient particles for analysis, followed by identification based on which conditions support growth, on reaction to specific antibodies, or based on nucleic acid hybridization (Gao and Moore, 1996). Biological growth can be omitted when the polymerase chain reaction (PCR) is used to amplify DNA, however, PCR requires sequence-specific primers, and is thus limited to known or suspected agents (Bai et al., 1997). For all these methods, considerable time is required, and the methods are useful for agents whose properties are known or suspected. Existing methods do not provide means for rapidly isolating and characterizing new infectious agents. Hundreds of infectious agents are known, and it is infeasible to have available reagents for an appreciable fraction of them.
Techniques for recovering infectious agents from blood, urine, and tissues have been previously developed based on centrifugation or filtration, but have not been widely used clinically (Anderson et al., 1966; Anderson et al., 1967). The highest resolution methods use rate zonal centrifugation to separate fractions based on sedimentation rate (measured in Svedberg units, S) and isopycnic banding density (measured in grams per mL or ρ). S-ρ separations have been used to isolate virus particles in a high state of purity from rat liver homogenates, and have been used to isolate the equivalent of approximately 20 virions per cell (Anderson et al., 1966). In these studies, virus particles were detected by light scattering and visualized by electron microscopy. The separations required complex special equipment not generally available, one or more days of effort, and they did not provide a definitive identification of the bacterial or viral species separated.
It is important to show that candidate infectious particles isolated by centrifugal methods actually contain nucleic acids. DNA and RNA in both active and fixed bacterial and viral particles have been stained with fluorescent dyes specific to nucleic acids, and observed and counted by fluorescent microscopy and flow cytometry. Many dyes are now known which exhibit little fluorescence in the free state, but become highly fluorescent when bound to nucleic acids. Some bind differentially to DNA or RNA or to different specific regions, and some show different emission spectra depending on whether bound to DNA or RNA. In this disclosure, dyes referred to are fluorescent dyes. By differential fluorescence spectroscopy ssDNA, dsDNA and RNA may be distinguished. See, Haugland, 1996; Mayor and Diwan, 1961; Mayor, 1961; Hobbie et al., 1977; Zimmerman, 1977; Perter and Feig, 1980; Paul, 1982; Suttle, 1993; Hirons et al., 1994; Hennes and Suttle, 1995; Hennes et al., 1995.
Isolated nucleic acid molecules of the dimensions found in bacteria and viruses have been counted and their mass estimated using fluorescence flow cytometry for molecules in solution, and epifluorescence microscopy of immobilized molecules (Hennes and Suttle, 1995, Goodwin et al., 1993). In both instances, the size of fragments produced by restriction enzymes can be estimated, and the molecules identified by reference to a database listing the sizes of fragments of known DNA molecules produced by different restriction enzymes (Hammond et al., U.S. Pat. No. 5,558,998; Jing et al., 1998).
Using specific fluorescently-labeled antibodies, specific identifications may also be made. These studies are time consuming, and require batteries of specific antibodies, together with epifluorescent microscopy or fluorimeters.
Matrix-Assisted Laser-Desorption-Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF-MS) currently allows precise measurements of the masses of proteins having molecular weights of over 50,000 daltons. Individual virion proteins have been previously studied by mass spectrometry (Siuzdak, 1998); however, resolution of complete sets of viral subunits from clinically relevant preparations of intact viruses, and the demonstration that precise measurements could be made of their individual masses, have not been previously reported. While single protein mass measurements can reliably identify many proteins, when a set of proteins from a virus or bacterial cell are known, detection of such a set provides more definitive identification. Methods are currently also being developed which allow partial sequencing of proteins or enzymatically produced peptide fragments and thus further increase the reliability of identifications. For MALDI-TOF-MS currently used methods require a picomole or more of protein, while electrospray mass spectrometry currently requires 5–10 femtomoles. The detection limits with mass spectrometry, especially MALDI, depend on getting a sample concentrated and on to a very small target area. Sensitivity will increase as ultramicro methods for concentrating and transferring ever smaller-volume samples are developed. See, Claydon et al., 1996; Fenselau, 1994; Krishmanurthy et al., 1996; Loo et al., 1997; Lennon and Walsh, 1997; Shevchenko et al., 1996; Holland et al., 1996; Liang et al., 1996.
Centrifugal methods for concentrating particles from large into small volumes have been in use for decades. Using microbanding centrifuge tubes which have a large cylindrical volume and cross section which tapers gradually in a centrifugal direction down to a small tubular section, particles may be concentrated or banded in a density gradient restricted to the narrow tubular bottom of the tube, or may be pelleted. The basic design of such tubes are well known by those skilled in the arts. See, Tinkler and Challenger, 1917; Cross, 1928; ASTM Committee D-2, 1951; Davis and Outenreath, U.S. Pat. No. 4,624,835; Kimura, U.S. Pat. No. 4,861,477; Levine et al., U.S. Pat. No. 5,342,790; Saunders et al., U.S. Pat. No. 5,422,018; Saunders, U.S. Pat. No. 5,489,396. The original tubes of this type were called Sutherland bulbs and were used to determine the water content of petroleum (The Chemistry of Petroleum and Its Substitutes, 1917, ASTM Tentative Method of Test for Water and Sediment by Means of Centrifuge, ASTM Designation: D 96-50T, 1947). Slight modifications of the basic design are described in U.S. Pat. Nos. 4,106,907; 4,624,835; 4,861,477; 5,422,018, 5,489,396. Such tubes have been made of glass or plastic materials, and the use of water or other fluids to support glass or plastic centrifuge tubes in metal centrifuge shields has long been well known in the art. However, centrifuge tubes disclosed in the prior art which include a shape similar to that of the microbanding centrifuge tubes of the instant invention could not withstand the centrifugal forces required to band viral particles in gradients. Conventional centrifuge tubes, or tubes derivative from the Sutherland design have been used for density gradient separations, and for separations in which wax or plastic barriers are used which position themselves between regions of different density to allow recovery of these fractions without mixing. There has been no previous discussion of barriers which prevent mixing of step gradient components at rest, but which barriers are centrifuged away from the gradient during rotation. Nor have tube closures for high-speed thin-walled swinging-bucket centrifuge tubes been described, whose exterior surfaces can be disinfected after the tubes are loaded.
The efficient stabilization of very shallow density gradients in centrifugal fields is well known, and is utilized in analytical ultracentrifugation to cause a sample layer to flow rapidly to the centripetal surface of a gradient without mixing using a synthetic boundary cell (Anderson, U.S. Pat. No. 3,519,400). Hence, light physical barrier disks between step gradient components can be moved away from the gradient by centrifugal force without appreciably disturbing the gradient, provided that they are made of porous, woven or sintered materials having a physical density less than that of the sample layer, such as polyethylene or polypropylene.
Many authors have noted that viruses and bacteria are often resistant to the actions of detergents and enzymes which will digest or dissolve contaminating particles of biological origin, and efforts have been made to classify infectious agents on the basis of their differential sensitivities. These differences have not previously and conveniently been incorporated in a method for detecting and quantifying infectious agents. See, Gessler et al., 1956; Theiler, 1957; Epstein and Hold, 1958; Kovacs, 1962; Planterose et al., 1962; Gard and Maaloe, 1959. Density differences between different species of virus and bacteria are well known, but have not been previously exploited for purposes of identification.
Infectious particles exhibit a wide range of isopycnic banding densities ranging from approximately 1.17 g/ml to 1.55 g/ml, depending on the type of nucleic acid present, and the ratios between the amount of nucleic acid, protein, carbohydrate, and lipid present. While such banding density differences are well known, no attempt has been previously made to systematically measure them and use the data to classify infectious agents.
The present invention is directed to an integrated system for concentrating, detecting and characterizing infectious agents using separations based on sedimentation rate and banding density, spectral analysis of emitted fluorescent light to distinguish DNA from RNA, differentiation of viral and bacterial particles from other particles by sedimentation through zones of solubilizing enzymes or reagents, determination of the isopycnic banding densities of infectious particles by reference to the positions of synthetic density standardization particles, particle detection using fluorescent dyes for DNA or RNA, further concentration of banded particles by pelleting, transfer of concentrated particles to mass spectrometer targets for protein mass determination and analysis, counting of concentrated particles by epifluorescent microscopy and fluorescence flow cytometry, and identification of bacterial or viral nucleic acids by restriction fragment length polymorphism analysis using either immobilized nucleic acid molecules, or ultrasensitive fluorescence flow cytometry. These methods are especially useful in characterizing biological samples which have low titres of virus and which contain viruses which are not culturable.
Furthermore, all current methods used to detect and characterize infectious agents, including use of fluorescent antibodies, detection of agent-associated enzymes, culture to increase agent mass, PCR amplification, restriction fragment length polymorphism analysis, hybridization to probes immobilized on chips, histochemical analysis, and all forms of microscopy including electron microscopy, are vastly improved by preconcentration of the microorganisms using the methods of the present invention.
These techniques have not previously been assembled into one operational system capable of routine field, hospital, and clinical laboratory use. The present application describes innovations and inventions which make such a system feasible. For work with potentially lethal agents, the system will be assembled in containment, and at least partially automated.